1Multi-touch (MT) Protocol
2-------------------------
3 Copyright (C) 2009-2010 Henrik Rydberg <rydberg@euromail.se>
4 5 6Introduction
7------------
8 9In order to utilize the full power of the new multi-touch and multi-user
10devices, a way to report detailed data from multiple contacts, i.e.,
11objects in direct contact with the device surface, is needed. This
12document describes the multi-touch (MT) protocol which allows kernel
13drivers to report details for an arbitrary number of contacts.
14 15The protocol is divided into two types, depending on the capabilities of the
16hardware. For devices handling anonymous contacts (type A), the protocol
17describes how to send the raw data for all contacts to the receiver. For
18devices capable of tracking identifiable contacts (type B), the protocol
19describes how to send updates for individual contacts via event slots.
20 21 22Protocol Usage
23--------------
24 25Contact details are sent sequentially as separate packets of ABS_MT
26events. Only the ABS_MT events are recognized as part of a contact
27packet. Since these events are ignored by current single-touch (ST)
28applications, the MT protocol can be implemented on top of the ST protocol
29in an existing driver.
30 31Drivers for type A devices separate contact packets by calling
32input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
33event, which instructs the receiver to accept the data for the current
34contact and prepare to receive another.
35 36Drivers for type B devices separate contact packets by calling
37input_mt_slot(), with a slot as argument, at the beginning of each packet.
38This generates an ABS_MT_SLOT event, which instructs the receiver to
39prepare for updates of the given slot.
40 41All drivers mark the end of a multi-touch transfer by calling the usual
42input_sync() function. This instructs the receiver to act upon events
43accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
44of events/packets.
45 46The main difference between the stateless type A protocol and the stateful
47type B slot protocol lies in the usage of identifiable contacts to reduce
48the amount of data sent to userspace. The slot protocol requires the use of
49the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
50the raw data [5].
51 52For type A devices, the kernel driver should generate an arbitrary
53enumeration of the full set of anonymous contacts currently on the
54surface. The order in which the packets appear in the event stream is not
55important. Event filtering and finger tracking is left to user space [3].
56 57For type B devices, the kernel driver should associate a slot with each
58identified contact, and use that slot to propagate changes for the contact.
59Creation, replacement and destruction of contacts is achieved by modifying
60the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
61is interpreted as a contact, and the value -1 denotes an unused slot. A
62tracking id not previously present is considered new, and a tracking id no
63longer present is considered removed. Since only changes are propagated,
64the full state of each initiated contact has to reside in the receiving
65end. Upon receiving an MT event, one simply updates the appropriate
66attribute of the current slot.
67 68Some devices identify and/or track more contacts than they can report to the
69driver. A driver for such a device should associate one type B slot with each
70contact that is reported by the hardware. Whenever the identity of the
71contact associated with a slot changes, the driver should invalidate that
72slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is
73tracking more contacts than it is currently reporting, the driver should use
74a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
75being tracked by the hardware at that moment. The driver should do this by
76explicitly sending the corresponding BTN_TOOL_*TAP event and setting
77use_count to false when calling input_mt_report_pointer_emulation().
78The driver should only advertise as many slots as the hardware can report.
79Userspace can detect that a driver can report more total contacts than slots
80by noting that the largest supported BTN_TOOL_*TAP event is larger than the
81total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
82 83Protocol Example A
84------------------
85 86Here is what a minimal event sequence for a two-contact touch would look
87like for a type A device:
88 89 ABS_MT_POSITION_X x[0]
90 ABS_MT_POSITION_Y y[0]
91 SYN_MT_REPORT
92 ABS_MT_POSITION_X x[1]
93 ABS_MT_POSITION_Y y[1]
94 SYN_MT_REPORT
95 SYN_REPORT
96 97The sequence after moving one of the contacts looks exactly the same; the
98raw data for all present contacts are sent between every synchronization
99with SYN_REPORT.
100 101Here is the sequence after lifting the first contact:
102 103 ABS_MT_POSITION_X x[1]
104 ABS_MT_POSITION_Y y[1]
105 SYN_MT_REPORT
106 SYN_REPORT
107 108And here is the sequence after lifting the second contact:
109 110 SYN_MT_REPORT
111 SYN_REPORT
112 113If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
114ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
115last SYN_REPORT will be dropped by the input core, resulting in no
116zero-contact event reaching userland.
117 118 119Protocol Example B
120------------------
121 122Here is what a minimal event sequence for a two-contact touch would look
123like for a type B device:
124 125 ABS_MT_SLOT 0
126 ABS_MT_TRACKING_ID 45
127 ABS_MT_POSITION_X x[0]
128 ABS_MT_POSITION_Y y[0]
129 ABS_MT_SLOT 1
130 ABS_MT_TRACKING_ID 46
131 ABS_MT_POSITION_X x[1]
132 ABS_MT_POSITION_Y y[1]
133 SYN_REPORT
134 135Here is the sequence after moving contact 45 in the x direction:
136 137 ABS_MT_SLOT 0
138 ABS_MT_POSITION_X x[0]
139 SYN_REPORT
140 141Here is the sequence after lifting the contact in slot 0:
142 143 ABS_MT_TRACKING_ID -1
144 SYN_REPORT
145 146The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
147message removes the association of slot 0 with contact 45, thereby
148destroying contact 45 and freeing slot 0 to be reused for another contact.
149 150Finally, here is the sequence after lifting the second contact:
151 152 ABS_MT_SLOT 1
153 ABS_MT_TRACKING_ID -1
154 SYN_REPORT
155 156 157Event Usage
158-----------
159 160A set of ABS_MT events with the desired properties is defined. The events
161are divided into categories, to allow for partial implementation. The
162minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
163allows for multiple contacts to be tracked. If the device supports it, the
164ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
165of the contact area and approaching tool, respectively.
166 167The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
168looking through a window at someone gently holding a finger against the
169glass. You will see two regions, one inner region consisting of the part
170of the finger actually touching the glass, and one outer region formed by
171the perimeter of the finger. The center of the touching region (a) is
172ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
173ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
174diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
175harder against the glass. The touch region will increase, and in general,
176the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
177than unity, is related to the contact pressure. For pressure-based devices,
178ABS_MT_PRESSURE may be used to provide the pressure on the contact area
179instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
180indicate the distance between the contact and the surface.
181 182 183 Linux MT Win8
184 __________ _______________________
185 / \ | |
186 / \ | |
187 / ____ \ | |
188 / / \ \ | |
189 \ \ a \ \ | a |
190 \ \____/ \ | |
191 \ \ | |
192 \ b \ | b |
193 \ \ | |
194 \ \ | |
195 \ \ | |
196 \ / | |
197 \ / | |
198 \ / | |
199 \__________/ |_______________________|
200 201 202In addition to the MAJOR parameters, the oval shape of the touch and finger
203regions can be described by adding the MINOR parameters, such that MAJOR
204and MINOR are the major and minor axis of an ellipse. The orientation of
205the touch ellipse can be described with the ORIENTATION parameter, and the
206direction of the finger ellipse is given by the vector (a - b).
207 208For type A devices, further specification of the touch shape is possible
209via ABS_MT_BLOB_ID.
210 211The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
212finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
213may be used to track identified contacts over time [5].
214 215In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
216implicitly handled by input core; drivers should instead call
217input_mt_report_slot_state().
218 219 220Event Semantics
221---------------
222 223ABS_MT_TOUCH_MAJOR
224 225The length of the major axis of the contact. The length should be given in
226surface units. If the surface has an X times Y resolution, the largest
227possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
228 229ABS_MT_TOUCH_MINOR
230 231The length, in surface units, of the minor axis of the contact. If the
232contact is circular, this event can be omitted [4].
233 234ABS_MT_WIDTH_MAJOR
235 236The length, in surface units, of the major axis of the approaching
237tool. This should be understood as the size of the tool itself. The
238orientation of the contact and the approaching tool are assumed to be the
239same [4].
240 241ABS_MT_WIDTH_MINOR
242 243The length, in surface units, of the minor axis of the approaching
244tool. Omit if circular [4].
245 246The above four values can be used to derive additional information about
247the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
248the notion of pressure. The fingers of the hand and the palm all have
249different characteristic widths.
250 251ABS_MT_PRESSURE
252 253The pressure, in arbitrary units, on the contact area. May be used instead
254of TOUCH and WIDTH for pressure-based devices or any device with a spatial
255signal intensity distribution.
256 257ABS_MT_DISTANCE
258 259The distance, in surface units, between the contact and the surface. Zero
260distance means the contact is touching the surface. A positive number means
261the contact is hovering above the surface.
262 263ABS_MT_ORIENTATION
264 265The orientation of the touching ellipse. The value should describe a signed
266quarter of a revolution clockwise around the touch center. The signed value
267range is arbitrary, but zero should be returned for an ellipse aligned with
268the Y axis of the surface, a negative value when the ellipse is turned to
269the left, and a positive value when the ellipse is turned to the
270right. When completely aligned with the X axis, the range max should be
271returned.
272 273Touch ellipsis are symmetrical by default. For devices capable of true 360
274degree orientation, the reported orientation must exceed the range max to
275indicate more than a quarter of a revolution. For an upside-down finger,
276range max * 2 should be returned.
277 278Orientation can be omitted if the touch area is circular, or if the
279information is not available in the kernel driver. Partial orientation
280support is possible if the device can distinguish between the two axis, but
281not (uniquely) any values in between. In such cases, the range of
282ABS_MT_ORIENTATION should be [0, 1] [4].
283 284ABS_MT_POSITION_X
285 286The surface X coordinate of the center of the touching ellipse.
287 288ABS_MT_POSITION_Y
289 290The surface Y coordinate of the center of the touching ellipse.
291 292ABS_MT_TOOL_X
293 294The surface X coordinate of the center of the approaching tool. Omit if
295the device cannot distinguish between the intended touch point and the
296tool itself.
297 298ABS_MT_TOOL_Y
299 300The surface Y coordinate of the center of the approaching tool. Omit if the
301device cannot distinguish between the intended touch point and the tool
302itself.
303 304The four position values can be used to separate the position of the touch
305from the position of the tool. If both positions are present, the major
306tool axis points towards the touch point [1]. Otherwise, the tool axes are
307aligned with the touch axes.
308 309ABS_MT_TOOL_TYPE
310 311The type of approaching tool. A lot of kernel drivers cannot distinguish
312between different tool types, such as a finger or a pen. In such cases, the
313event should be omitted. The protocol currently supports MT_TOOL_FINGER and
314MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
315drivers should instead use input_mt_report_slot_state().
316 317ABS_MT_BLOB_ID
318 319The BLOB_ID groups several packets together into one arbitrarily shaped
320contact. The sequence of points forms a polygon which defines the shape of
321the contact. This is a low-level anonymous grouping for type A devices, and
322should not be confused with the high-level trackingID [5]. Most type A
323devices do not have blob capability, so drivers can safely omit this event.
324 325ABS_MT_TRACKING_ID
326 327The TRACKING_ID identifies an initiated contact throughout its life cycle
328[5]. The value range of the TRACKING_ID should be large enough to ensure
329unique identification of a contact maintained over an extended period of
330time. For type B devices, this event is handled by input core; drivers
331should instead use input_mt_report_slot_state().
332 333 334Event Computation
335-----------------
336 337The flora of different hardware unavoidably leads to some devices fitting
338better to the MT protocol than others. To simplify and unify the mapping,
339this section gives recipes for how to compute certain events.
340 341For devices reporting contacts as rectangular shapes, signed orientation
342cannot be obtained. Assuming X and Y are the lengths of the sides of the
343touching rectangle, here is a simple formula that retains the most
344information possible:
345 346 ABS_MT_TOUCH_MAJOR := max(X, Y)
347 ABS_MT_TOUCH_MINOR := min(X, Y)
348 ABS_MT_ORIENTATION := bool(X > Y)
349 350The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
351the device can distinguish between a finger along the Y axis (0) and a
352finger along the X axis (1).
353 354For win8 devices with both T and C coordinates, the position mapping is
355 356 ABS_MT_POSITION_X := T_X
357 ABS_MT_POSITION_Y := T_Y
358 ABS_MT_TOOL_X := C_X
359 ABS_MT_TOOL_X := C_Y
360 361Unfortunately, there is not enough information to specify both the touching
362ellipse and the tool ellipse, so one has to resort to approximations. One
363simple scheme, which is compatible with earlier usage, is:
364 365 ABS_MT_TOUCH_MAJOR := min(X, Y)
366 ABS_MT_TOUCH_MINOR := <not used>
367 ABS_MT_ORIENTATION := <not used>
368 ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
369 ABS_MT_WIDTH_MINOR := min(X, Y)
370 371Rationale: We have no information about the orientation of the touching
372ellipse, so approximate it with an inscribed circle instead. The tool
373ellipse should align with the the vector (T - C), so the diameter must
374increase with distance(T, C). Finally, assume that the touch diameter is
375equal to the tool thickness, and we arrive at the formulas above.
376 377Finger Tracking
378---------------
379 380The process of finger tracking, i.e., to assign a unique trackingID to each
381initiated contact on the surface, is a Euclidian Bipartite Matching
382problem. At each event synchronization, the set of actual contacts is
383matched to the set of contacts from the previous synchronization. A full
384implementation can be found in [3].
385 386 387Gestures
388--------
389 390In the specific application of creating gesture events, the TOUCH and WIDTH
391parameters can be used to, e.g., approximate finger pressure or distinguish
392between index finger and thumb. With the addition of the MINOR parameters,
393one can also distinguish between a sweeping finger and a pointing finger,
394and with ORIENTATION, one can detect twisting of fingers.
395 396 397Notes
398-----
399 400In order to stay compatible with existing applications, the data reported
401in a finger packet must not be recognized as single-touch events.
402 403For type A devices, all finger data bypasses input filtering, since
404subsequent events of the same type refer to different fingers.
405 406For example usage of the type A protocol, see the bcm5974 driver. For
407example usage of the type B protocol, see the hid-egalax driver.
408 409[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
410[2] The list can of course be extended.
411[3] The mtdev project: http://bitmath.org/code/mtdev/.
412[4] See the section on event computation.
413[5] See the section on finger tracking.
414